In a known manner, the escapement of a mechanical timepiece movement, whether it is coaxial or a Swiss lever escapement, includes an escape wheel wherein the regularly spaced teeth undergo the shock of pallet-stones which are generally made of ruby.
An example of a coaxial escape system is presented in EP Patent No. 1 045 297. A mechanism of this type, as illustrated in FIG. 11, includes a wheel set formed of first 1 and second 11 escape wheels. The second escape wheel 11, also called an impulse pinion, is mounted on the same arbour 3. The angular position of teeth 13 of the second escape wheel 11 is indexed on the angular position of the teeth 8 of the first escape wheel 1.
The first escape wheel 1 takes the form of a conventional Swiss lever escape wheel. The second escape wheel 11 also acts as escape pinion 15 by meshing with an intermediate wheel 17, which has the advantage of reducing the height of the mechanism.
The roller 19 of the balance (not shown) carries an impulse-pin 21 and an impulse pallet-stone 23. The impulse-pin 21 is for driving lever 25, in a to-and-fro motion, via the fork 27 thereof, whereas impulse pallet-stone 23 is for cooperating with the teeth 8 of the first escape wheel 1.
Lever 25 includes an impulse pallet-stone 24 which cooperates with the teeth 13 of the second escape wheel 11. Lever 25 also includes two other, respectively entry and exit locking pallets 26, 28 which cooperate with the teeth 8 of the first escape wheel 1.
The design of this coaxial escapement requires very high precision, particularly as regards the relative alignment of the first 1 and second 11 escape wheels, which makes it difficult to manufacture. Moreover, generally speaking, the gear trains of a two level timepiece like the usual wheel-pinion type always need to have perfect indexing between them.